Will a shark drown if it stops moving?

Sharks take "doing the wave" to a whole new level. They swim by waving their body in side-to-side curves. It starts with the head turning first one way, then another. The movement ripples down their torpedo-shaped body, pushing the water away and providing forward propulsion. Last comes the tail, the fins shaped to allow them to move quickly and achieve immense liftoff or propulsive downward thrust. Obviously a shark has to swim to catch its prey, meet its mates and avoid its predators, but does a shark have to swim just to stay alive?

You may have heard that a shark will drown if it stops moving, an idea that has been cited everywhere from biology textbooks to "Ripley's Believe It or Not!" [source: Bennetta]. This theory came about by comparing sharks to bony fish, which have many more muscles around the breathing apparatuses, the gills.

To understand why this distinction is important, let's take a look at how sharks and other fish breathe. To breathe, sharks must remove oxygen from the water around them. The water enters the shark's mouth (the shark's nose is used exclusively for smell) and flows over the gills. Inside the gills are hundreds of feathery gill filaments. Each filament in turn has thousands of leaflike lamellae, or flaps, which contain blood vessels. The blood absorbs the oxygen from the incoming water, and the excess water flows back out the shark's body through gill slits. Sharks have five to seven pairs of gill slits, depending on the species.

With this method, sharks can extract about 80 percent of oxygen out of the meager 1 percent of oxygen that's present in the water; to compare, humans have 21 percent of oxygen available in the air, but take in only about 25 percent [source: Parker]. To maintain a steady flow, though, the shark constantly needs to be taking in water.

But does it have to constantly swim to take in this water? Scientists thought so because other fish seemed to have the equipment to actively pump the water through their mouth and over their gill slits, whereas sharks looked less developed. But how to account for sharks that don't seem to swim at all, like the angel sharks and nurse sharks we might see at aquariums? It turns out that not all sharks have to stay moving to breathe. Find out which sharks are constant swimmers on the next page.

Shark Breathing: Buccal Pumping and Ram Ventilation

The oldest sharks, the modern sharks' ancestors, didn't have to constantly swim to breathe. Rather, they all pumped water through their mouth and over their gills. This method is known as buccal pumping, named for the buccal, or cheek, muscles that pull the water into the mouth and over the gills. Many sharks retain this method today, such as nurse sharks, angel sharks and carpet sharks, also known as wobbegongs. Skates and rays, the shark's cousins, also breathe this way. These species tend to spend most of their time lying on the bottom of the ocean floor.

In addition to an inactive lifestyle, there are some additional bodily differences that allow these sharks to breathe by buccal pumping. For example, many of the sharks that practice this method are dorsoventrally flattened (or squashed along the length of its back), like the angel shark. They have stronger muscles in the face. These sharks might also have a more prominent spiracle, which is a tube behind the eyes. When a shark is buried at the bottom of the ocean floor and can't breathe through its mouth, the spiracle acts like a mouth by pulling in water. The water then exits through the gill slits.

As sharks evolved and became more active, however, this method of pumping became secondary. It was simply more energy efficient to take in water while swimming, in effect "ramming" the water into the mouth and letting it flow out through the gills slits. This method of breathing is known as ram ventilation. Most sharks can alternate between buccal pumping and ram ventilation, depending on what they're doing. When they start swimming fast enough to force the water in more quickly than they could pump it, then they stop pumping. The sand tiger shark is an example of a shark that switches back and forth.

Some sharks, however, have completely lost the ability to breathe by buccal pumping, and these are the sharks that will indeed drown if they stop swimming and ramming water. These sharks are known as obligate ram breathers (or obligate ram ventilators); only about two dozen of the 400 identified shark species are required to maintain this forward swimming motion [source: Bennetta]. These include the great white shark, the mako shark, the salmon shark and the whale shark.

Do these obligate ram breathers ever get a break? Aren't they tired? Read on to see if they can catch a nap and what can sink them on the next page.

Even in oxygen-rich water, a net will impair a shark's movement so it can't breathe.

Shark Sleeping

It might seem tiring to us humans to think about any sort of perpetual movement as a way to survive; we all like crashing on the couch every now and then. But as it turns out, it's more work for these sharks to remain still than it is to swim. In a study of lemon sharks, which switch between breathing methods, juveniles breathed 6 percent more efficiently when moving than when resting, even when resting so that the current allowed the water to flow directly into their mouths [source: Morrissey and Gruber].

This fact may help to explain what happens when sharks face the danger of hypoxia, or a deficiency of available oxygen. Sharks that breathe by buccal pumping increase the force of the pumping to try to bring in more oxygen while reducing their other activity to conserve energy. Obligate ram breathers, however, increase their energy, swimming faster and opening their mouth wider [source: Carlson and Parsons]. It may seem counterintuitive to speed up when faced with less oxygen, but that just may be more energy efficient for these sharks.

That's not to say these sharks don't catch a break every now and then. For obvious reasons, it can be hard to keep track of a shark that's constantly swimming, so it's difficult for scientists to know how or when they rest. An experiment with a small shark, the spiny dogfish, indicated that swimming is coordinated by the spinal cord, not by the brain, so sharks may be able to shut down their brain and rest while still swimming [source: Martin].

Sharks in need of some rest may also take advantage of the factors that affect the amount of oxygen in the water, such as salinity, temperature and even time of day. In the 1970s, scientists investigated what came to be known as the Caves of the Sleeping Sharks in Isla Mujeres, Mexico. Inside the caves were motionless reef sharks, which are normally obligate ram ventilators. The scientists determined that the water in the caves had an extremely high amount of oxygen and reduced salinity. These conditions likely made it easier for even these sharks to breathe without moving.

They may not have been asleep like humans (their eyes were open, for one thing), but it does appear that sharks can get some rest. Many other reef sharks also have been observed motionless at the bottom, even outside of caves [source: Martin]. Scientists still aren't exactly sure how they can do this.

There are some unfortunate incidences where the shark's coping mechanisms just won't work, however, and they're usually caused by man. Illegal finning, which occurs when a shark's fin is cut off and the shark is thrown back to sea, sometimes still alive, usually results in the shark's eventual drowning. Getting caught in fishing nets can cause death. Sometimes just being transported to an aquarium spells doom for obligate ram ventilators.

To learn more about sharks and how they work, take a look at the links on the next page.